Posse

A group of men deputized by The Sheriff or US Marshal to assist in the pursuit of justice, such as capturing fugitives.

The word comes from the phrase "posse comitatus", roughly "the power of the county." In the old days, it would sometimes be necessary for a sheriff to get a lot of manpower very quickly to deal with a crisis. To this end, he could essentially draft any man handy (with certain restrictions) into a posse to handle the situation. The process will often include the line "I hereby deputise you".

After 1878, it was illegal to use military personnel in a posse. In more modern times, many jurisdictions have banned the formation of posses, but sheriffs seldom have need of them due to improved communications and manpower.

Note that if it is not legally convened by a sheriff or marshal, the group of men is not a "posse", even if they call themselves that. They're just a mob or gang.

In modern slang, a "posse" is just a group of people who hang out with each other all the time; see Girl Posse and Production Posse for examples of this sort of "posse."

Examples of Posse include:

Film

Posse was a 1993 movie by Mario Van Peebles that has a gang formed up of black Civil War veterans retuning home to right wrongs. Unfortunately, they can't be a legal posse as The Sheriff is one of the bad guys.
Subverted in the Gary Cooper film,High Noon - Marshal Kane tries to gather a posse to take down revenge-seeking Outlaw Frank Miller, but he's forced to fight alone when none of the eligible townsfolk will help him.
Butch Cassidy and the Sundance Kid.
- The town marshal tries to get a posse together to pursue the Hole in the Wall gang after a train robbery, but his lack of charisma, the gang's lethal reputation and the intervention of a ~~Snake Oil Salesman~~ bicycle salesman foil him.
- During the second train robbery, the gang is attacked by a expert posse specially formed by "Mr. E. H. Harriman of the Union Pacific Railroad" to hunt down Butch and the Sundance Kid. This pursuit eventually drives Our Heroes to Bolivia, and their doom.
In Young Guns, loosely based on the Real Life adventures of Billy the Kid, Billy and the other "Regulators" are deputized as a posse through political influence, but quickly lose that status when they abuse their power. The sequel, Young Guns 2, has a legitimate posse formed by Sheriff Pat Garrett to pursue Billy's gang.

Literature

The Ox-Bow Incident by Walter Van Tilburg Clark, a book later turned into a movie, has a posse illegally drafted by a deputy sheriff. By the end, the sheriff forms a genuine posse.
In Edgar Pangborn's short story "Tiger Boy", which is set in the same post-holocaust world as his novel Davy, some villagers form a posse (legal status unclear) to apprehend the title character whom they consider to be a demon with an animal familiar. Actually he's just some kid who wanders around with a half-tame tiger looking for a friend, and he finds one in the form of an intelligent but mute village boy. The two of them plan to run away together, but with the posse on their trail it can't end well.
In 1632 when Grantsville finds a band of soldiers raping and pillaging local farmsteads the police chief deputizes a posse to pursue them. Of course they are only mildly desirous of capturing the fugitives-they aren't interested in red tape at the momment

Live Action TV

Bonanza had a posse formed at least three times a season. With 15 seasons, that's a lot of posse forming.
Supernatural has a Wild West themed episode that involves the formation of a posse. Dean is very happy about that. He loves the posse. He's a posse magnet.

Music

The song "One Hour Ahead of the Posse" by Burl Ives, which told the story of a murderer trying to reach the Rio Grande river and sanctuary in Mexico.

Tabletop Games

Inquisitors in Warhammer 40,000 have the power to Flashed Badge Hijack anything up to and including entire planetary populations, but most just run around with a small retinue that serves the same purpose as a Posse.
- Plus, as detailed in Dark Heresy, however many cells of agents here and there they have time to organize, train, supply and keep somewhat controlled (standards for all this may vary wildly even between cells of the same boss) but those can have authority anywhere in range from Throne Agent badge(s) to completely off-the-books undercover with Plausible Deniability, depending on the task. Either way, most Inquisitors don't wave their Rosette most of the time - keeping low profile and using the henchmen already proven on lesser missions usually works better.

Real Life

The custom evolved from laws regarding similar practices in Britain. For instance on the Anglo-Scottish border, it was perfectly legal to pursue a raider band within a week after an attack for that was regarded as an extension of self-defense rather then a crime. If the pursuers actually found them with the spoils of the raid (that's the origin of the term "red-handed") it was even legal to execute them summarily. This rather abrupt idea of justice was to some degree limited by the fact that the robbers might have relations who would be very annoyed.